Artificial gravity and abort scenarios via tethers for human missions to Mars

Minimum-mass tether designs are developed for a spinning human transport that not only provides artificial gravity, but also the potential for free-return aborts. The investigation reveals that severing the tether can provide a propellant-free boost to return astronauts to Earth in the event of an aborted landing on Mars. Earth–Mars–Earth, Earth–Mars–Venus–Earth, and Earth–Venus–Mars–Earth trajectories requiring little, or no, velocity change after departure from Earth, are examined. The investigation covers trajectories with launch opportunities between 2014 and 2030, launch hyperbolic excess speeds of up to 4.5 km/s and total flight times of less than 1000 days. We identify propellant-free abort scenarios in every Earth–Mars synodic period (2.14 years) with mission configurations that closely match NASA’s design reference missio

A declarative approach for flexible business processes management

Abstract. Management of dynamic processes in an important issue in rapidly changing organizations. Workflow management systems are systems that use detailed process models to drive the business processes. Current business process modelling languages and models are of imperative nature – they strictly prescribe how to work. Systems that allow users to maneuver within the process model or even change the model while working are considered to be the most suitable for dynamic processes management. However, in many companies it is not realistic to expect that end-users are able to change their processes. Moreover, the imperative nature of these languages forces designer to over-specify processes, which results in frequent changes. We propose a fundamental paradigm shift for flexible process management and propose a more declarative approach. Declarative models specify what should be done without specifying how it should be done. We propose the ConDec language for modelling and enacting dynamic business processes. ConDec is based on temporal logic rather than some imperative process modelling language. Key words: Workflow management, declarative model specification, dynamic workflow, flexibility, temporal logic

Constraint-based workflow models : change made easy

The degree of flexibility of workflow management systems heavily influences the way business processes are executed. Constraint-based models are considered to be more flexible than traditional models because of their semantics: everything that does not violate constraints is allowed. Although constraint-based models are flexible, changes to process definitions might be needed to comply with evolving business domains and exceptional situations. Flexibility can be increased by run-time support for dynamic changes – transferring instances to a new model – and ad-hoc changes – changing the process definition for one instance. In this paper we propose a general framework for a constraint-based process modeling language and its implementation. Our approach supports both ad-hoc and dynamic change, and the transfer of instances can be done easier than in traditional approaches